We have previously shown that peripheral reticulocytes of the rabbit synthesize only a limited number of membrane proteins; the major peptides appear to be made in earlier, nucleated forms in the bone marrow. We have utilized methods available in the literature for separating red cell precursors in the bone marrow into less and more mature fractions containing predominantly nucleated forms. In vivo studies indicate that the major membrane peptides, especially those of high molecular weight, are made predominantly by the most immature red cell precursors. We have devised a method for preparing highly purified plasma membranes from these cells. We now propose to carry out studies in vitro of membrane peptide and glycoprotein synthesis at the different stages of red cell maturation. Mixed marrow cells enriched in erythrocyte precursors, obtained from animals made anemic by phenylhydrazine or repeated bleeding, will be labelled in vitro with (H3) leucine and/or (C14) glucosamine. They will then be separated into fractions representing predominantly early or late precursors, isolated plasma membranes prepared, and individual peptide (or glycoprotein) synthesis determined by polyacrylamide gel electrophoresis and gel scanning and slicing. We hope to show the changing pattern of surface protein synthesis associated with red cell maturation from early proerythroblast to late normoblast stages. BIBLIOGRAPHIC REFERENCES: Koch, P.A., Gardener, F.H., Gartrell, J.E. Jr., and Carter, J.R. Jr.: Biogenesis of Erythrocyte Membrane Proteins: in vitro Studies with Rabbit Reticulocytes. Biochem. Biophys. Acta 389:177-187, (1975). Koch, P.A., Gartrell, J.E. Jr., Gardner, F.H., and Carter, J.R. Jr. Biogenesis of Erythrocyte Membrane proteins: in vivo Studies in Anemic Rabbits. Biochem. Biophys. Acta 389:162-176, (1975).